Well Matty obviously you have thought about the standards thing, so I would say a good idea would be to get something that can show in another class (so your hams wouldn't be against each other in a class) but it would also be useful to get something that could also complement your original line.
Just as an example, if you chose golden for your first line, then goldens carrying black often do better for showing as the black will darken the cheekflashes etc.
Your second line then could also be a black line. You could then cross the two lines when you wanted, say for example you wanted to improve the type of your blacks and you have a golden carrying black that has great type.
Is it cream you are going for for your first line? If so, then sable or mink (if standardised, and this could take some years if it happens) would be good as again, the genetics are compatible and you could get a mix of good pups in both litters.
Or you could just decide to add in an extra to your original line, like satins (show in a different class) or banded/roan (patterned class ) although patterning is a hard thing to get right for showing.
The one thing I would advise (learnt through hard experience myself) is to concentrate firstly on really getting your first line going and healthy and then see where you want to go from there.
Just as advice i havent taken myself
I would tend to steer clear of the harder gene types, such as dark grey in early days, and I would also be wary of the silver grey. Though they are lovely, this gene is highly dominant, so you need to be careful breeding this into other varieties.
If it was just for personal choice, I would go mink and I do wish it was standard. I really love minks, and the way the colour changes and deepens as the ham gets older is lovely. But until it is, if showing is your plan, then I wouldnt have this as a full second line.